The principle governing the operation of most current magnetic read heads is the change of resistivity of certain materials in the presence of a magnetic field (magneto-resistance or MR). Magneto-resistance can be significantly increased by means of a structure known as a spin valve or SV. The resulting increase (known as Giant Magneto-Resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of their environment.
The key elements of a spin valve are a low coercivity (free) ferromagnetic layer, a non-magnetic spacer layer, and a high coercivity ferromagnetic layer. The latter is usually formed out of a soft ferromagnetic layer that is pinned magnetically by a nearby layer of antiferromagnetic material. Additionally, a synthetic antiferromagnet (formed by sandwiching an antiferromagnetic coupling layer between two antiparallel ferromagnetic layers) may be used to replace the ferromagnetic pinned layer. This results in an increase in the size of the pinning field so that a more stable pinned layer is obtained. We will refer to it as a synthetically pinned device.
When the free layer is exposed to an external magnetic field, the direction of its magnetization is free to rotate according to the direction of the external field. After the external field is removed, the magnetization of the free layer will stay at a direction, which is dictated by the minimum energy state, determined by the crystalline and shape anisotropy, current field, coupling field and demagnetization field. If the direction of the pinned field is parallel to the free layer, electrons passing between the free and pinned layers, suffer less scattering. Thus, the resistance at this state is lower. If, however, the magnetization of the pinned layer is anti-parallel to that of the free layer, electrons moving from one layer into the other will suffer more scattering so the resistance of the structure will increase. The change in resistance of a spin valve is typically 8–20%.
Most GMR devices have been designed so as to measure the resistance of the free layer for current flowing parallel to the film's plane. However, as the quest for ever greater densities continues, devices that measure current flowing perpendicular to the plane (CPP) have begun to emerge. For devices depending on in-plane current, the signal strength is diluted by parallel currents flowing through the other layers of the GMR stack, so these layers should have resistivities as high as possible while the resistance of the leads into and out of the device need not be particularly low. By contrast, in a CPP device, the resistivity of both the leads and the other GMR stack layers dominate and should be as low as possible.
A device that is particularly well suited to the CPP design is the magnetic tunneling junction (MTJ) in which the layer that separates the free and pinned layers is a non-magnetic insulator, such as alumina or silica. Its thickness needs to be such that it will transmit a significant tunneling current. The principle governing the operation of the MTJ is the change of resistivity of the tunnel junction between two ferromagnetic layers. When the magnetization of the two ferromagnetic layers is in opposite directions, the tunneling resistance increases due to a reduction in the tunneling probability. The change of resistance is typically about 40%.
Although the layers enumerated above are all that is needed to produce the GMR or MTJ effects, additional problems remain. In particular, there are certain noise effects associated with such a structure. Magnetization in a layer can be irregular because of reversible breaking of magnetic domain walls, leading to the phenomenon of Barkhausen noise. The solution to this problem has been to provide a device structure conducive to ensuring that the free layer is a single domain and to ensure that the domain configuration remains unperturbed after processing and fabrication steps and under normal operation. For CIP devices this was most commonly accomplished by giving the structure a permanent longitudinal bias provided by two opposing permanent magnets located at the sides of the device.
As track widths grow very small (<0.2 microns), the above biasing configuration has been found to no longer be suitable since the strong magnetostatic coupling at the track edges also pins the magnetization of the free layer which drastically reduces the SV or MTJ sensor sensitivity. The solution to this problem that has been adopted by the prior art is illustrated in FIG. 1 for the case of an MTJ but is similarly applicable to an SV.
Seen in FIG. 1 is a bottom magnetic shield 11 on which rests a lower contact layer 12. The so-called pillar structure begins with pinned layer 14 which rests on anti-ferromagnetic (pinning) layer 13. Free layer 16 is separated from layer 14 by insulating layer 15 (which would be a non-magnetic metal layer if this were an SV). Longitudinal stabilization of free layer 16 is effected through a second (weaker) antiferromagnetic layer 18. To reduce the pinning effects of layer 18 on free layer 16, a very thin non-magnetic layer 17 is inserted between 16 and 18 to reduce the exchange coupling between them. The rest of the structure is routine—upper contact layer 19 and top magnetic shield 20.
The main problem associated with the design shown in FIG. 1 is that, because of the very small width of the free layer, its tendency to demagnetize is very strong so the intentionally weak coupling between it and layer 18 is often insufficient to provide the degree of longitudinal stabilization that is needed.
The present invention discloses an alternative design in which this problem is significantly reduced.
A routine search of the prior art was performed with the following references of interest being found:
In U.S. Pat. No. 5,883,763, Yuan et al. show a CPP GMR while U.S. Pat. No. 6,219,212 B1 (Gill et al.) discloses an MTJ structure. Sakakima shows both MTJ and CPP structures in U.S. Pat. No. 6,084,752 and, in U.S. Pat. No. 6,249,407 B1, Aoshima et al. show a CPP MR structure.